Corrugated board is made by attaching one or more flat sheets of paper to a medium, a wavy or fluted section of paper. The medium may be formed using any one of a variety of conventional and well-known corrugation processes that provide any one of a variety of flute sizes. In applications that call for relatively small flute sizes, corrugating the medium sometimes results in cracks forming in the paper. In addition, when only one flat sheet is attached to the medium, such as in the case of certain types of gift boxes, then the cracks become visible and aesthetically unappealing. Many solutions to this problem have been suggested in the prior art, as described below.
U.S. Pat. No. 3,687,767 to Reisman et al. discloses preventing cracks in corrugating medium by first treating the paper with a resin and then drying the resin. The paper is then corrugated and, following that, is heat treated to cure the resin. This curing step is shown in the flow chart of FIG. 1 of Reisman et al. in the fourth step from the bottom. Similarly, U.S. Pat. No. 3,173,829 to Thier et al. is directed to a process in which an additive is provided to the paper pulp prior to the sheet forming and drying process. After the paper has been manufactured, the additive (bonding agent) is activated by heat and/or pressure.
Both the Reisman et al. and Thier et al. schemes require an extra curing or activating step after the paper has been manufactured. This may be impractical in some situation such as, for example, a large corrugated board manufacturing operation that is not readily equipped to incorporate this extra step into their process. In addition, in cases where curing or activation is provided by heat, an inadvertent and premature exposure of the product to heat may cause undesirable results.
U.S. Pat. No. 3,033,708 to McKee, U.S. Pat. No. 3,307,994 to Scott, Jr., U.S. Pat. No. 3,308,006 to Kresse et al., U.S. Pat. No. 3,518,216 to Harvey et al., U.S. Pat. No. 3,659,772 to Dorsey et al., and U.S. Pat. No. 4,038,122 to DeLigt are all directed to applying an additive to the finished paper product during, or just prior to, corrugation in order to prevent cracks during the corrugation process. However, applying the additive to the finished paper product adds another step and requires extra machinery located either at the corrugator or at the paper manufacturer. In addition, applying an additive to the finished paper product can create difficulties with respect to handling any excess additive that does not adhere to the paper.
U.S. Pat. No. 3,109,769 to Martin, U.S. Pat. No. 3,119,731 to Strole et al., U.S. Pat. No. 3,298,902 to Osborg et al., and U.S. Pat. No. 3,525,668 to Goldstein et al. all appear to be directed to providing an additive to the pulp during the paper manufacturing process. Although this arguably avoids the need for extra equipment to apply the additive, it does result in a relatively large amount of additive being used since the additive ends up being applied throughout the entirety of the paper, rather than just at the surfaces.
In chapter 7 of a 1995 textbook titled "Paper Coating Additives" published by Tappi Press of Atlanta, Ga., the author suggests that dilutions of oxidized polyethylene emulsions have been applied at a size press or through surface spraying to lower surface coefficient of friction. The author goes on to state that spraying the oxidized polyethylene emulsions in the manufacture of corrugating medium decreases cracking during the corrugation step. However, since the author suggests only spraying to reduce cracking (as opposed to applying the emulsion with a size press to improve the coefficient of friction), this solution is similar to the solutions in references discussed above that disclose applying an additive after the paper manufacturing process is complete or just prior to corrugation.
It is desirable, therefore, to prevent cracks in the corrugated bound medium in an economical and feasible manner.